Strain wave gearing are mainly lubricated by an oil and a grease. A strain wave gearing disclosed in Patent document 1 is provided with a mechanism for supplying a grease from a grease reservoir. In a strain wave gearing described in Patent document 2, an oil tank is arranged inside a cup-shaped externally toothed gear so that the oil tank rotates integrally with the externally toothed gear, and an oil is supplied to portions where lubrication is required by making use of centrifugal force. In a power transmission device disclosed in Patent document 3, for the purpose of reducing friction and wear between sliding members, sliding surfaces are set to have a prescribed surface roughness, and are coated with a carbon-based film, and grease lubrication is applied between the sliding surfaces coated with the carbon-based film.